Materials

Question 1

What is density?

Answer 1

Mass per unit volume.

Question 2

How does the density of an object determine if it floats or sinks?

Answer 2

A solid object will float on a fluid if it has a lower density than the fluid.

Question 3

What is Archimedes principle?

Answer 3

When a body is completely or partially immersed in a fluid, it experiences an upthrust equal to the weight of the fluid it has displaced.

Upthrust = weight of fluid displace

Question 4

What is pressure and what is it measured in?

Answer 4

The amount of force acting per unit area.

P = F/A.

Measured in Pascals where 1Pa = 1Nm^-2

Question 5

What is the equation for viscous drag (Stoke’s law)?

Answer 5

F = 6πrηv

“F” is the viscous drag.

“r” is the radius of the ball.

“η” is the viscosity of the liquid.

“v” is the terminal velocity of the ball.

Question 6

What happens to the forces on a ball when it is travelling at terminal velocity?

Answer 6

W = F + U

Question 7

What is viscous drag?

Answer 7

The friction between the surface of an object and the fluid it is moving through and is always acting in the opposite direction to the movement.

Question 8

What factors affect viscous drag?

Answer 8

Viscous drag is affected by the viscosity of fluid and viscosity is affected by temperature and state.

Liquids are less viscous at higher temperatures whereas gases are more viscous at higher temperatures.

Question 9

Describe laminar flow around an object.

Answer 9

Laminar flow is a flow pattern where all the parts of the fluid are flowing in the same direction where the layers in the fluid do not mix.

Question 10

What conditions usually cause a laminar flow pattern?

Answer 10

It occurs when a fluid is flowing slowly, or when an object is moving slowly through a fluid.

Question 11

Describe turbulent flow.

Answer 11

It is a flow pattern where parts of the fluid get mixed up.

Question 12

What conditions usually cause a turbulent flow pattern?

Answer 12

It usually occurs when a fluid is flowing quickly or when an object is moving quickly through the fluid.

Question 13

What is Stoke’s law?

Answer 13

An expression for the resisting force on a sphere moving through a viscous fluid.

Question 14

Under what conditions does Stoke’s law apply?

Answer 14

It is directly proportional to the velocity and radius of the sphere.

Only applies to small, spherical objects moving slowly with laminar flow.

Does not apply to turbulent flow.

Question 15

Define Hooke’s law in words, and in an equation.

Answer 15

The force needed to stretch a spring is directly proportional to the extension of the spring from its original length.

F = k x

Force = spring constant x extension

Question 16

What must be true in order for an object to be stretched or compressed?

Answer 16

A pair of forces must be acting on the object, in different directions.

Question 17

Give four objects that Hooke’s law applies to.

Answer 17

Spring
Guitar string
Copper
Rubber (only a small amount)

Question 18

Define the limit of proportionality.

Answer 18

Up to this point the material obeys Hooke’s law, extension is proportional to the force applied.

Question 19

Define the elastic limit.

Answer 19

The final point where the material will return to its original length if the stress is removed. The original shape of the material stays the same. Material acts plastically beyond its elastic limit

Question 20

Define elastic deformation.

Answer 20

A temporary change of shape that is self-reversing after the force is removed, so that the object returns to its original shape

Question 21

Explain elastic deformation, in terms of atoms being under tension.

Answer 21

When a material is put under tension, the atoms are pulled apart from one another

Atoms can move slightly relative to their equilibrium positions, without changing position in the material

Once the load is removed, the atoms return to their equilibrium distance apart

Question 22

Define plastic deformation.

Answer 22

The material will not return to its original shape as it is permanently stretched.

Question 23

Explain plastic deformation in terms of atoms under tension.

Answer 23

Some atoms in the material move position relative to one another

When the load is removed, the atoms don’t return to their original positions

Question 24

Describe what happens at the yield point.

Answer 24

The material starts to stretch without any extra load.

Question 25

What does the gradient of force-extension graph give?

Answer 25

The spring constant.

Question 26

What does the area under a force-extension graph give?

Answer 26

Work done

Question 27

Before the elastic limit is exceeded, where is the work done in stretching/compressing transferred to?

Answer 27

The work done is transferred to permanently deforming the wire.

Question 28

What does a force-compression graph look like?

Answer 28

A straight positive line through the origin as extension is directly proportional to compression.

Question 29

Define (tensile/compressive) stress. Give the formula.

Answer 29

Stress: this is the force applied per cross sectional area.

Pa=F/A

“Pa” is pascals (unit for pressure).

Question 30

Define (tensile/compressive) strain. Give the formula.

Answer 30

A ratio of the change in length divided by original length. NO UNITS

Question 31

What is breaking stress?

Answer 31

Maximum stress that material can withstand before fracturing.

Question 32

What is UTS (ultimate tensile stress)

Answer 32

The maximum force per unit cross sectional area that can be applied before the sample breaks.

Question 33

What condition does breaking stress and UTS depend on?

Answer 33

Tempurature

Question 34

What is the Young modulus of a material? What are its units.

Answer 34

It is a measure of the stiffness of a material

Youngs modulus, “E”, is stress / dtrain

Measured in pascals or Nm^-2

Question 35

Can the value of Young modulus change?

Answer 35

It works up to the limit of proportionality.

Question 36

What property of a material does Young modulus give you a measure?

Answer 36

Stiffness.

It is a measure of how difficult it is to stretch or compress an object.

A stiff material has a large Young modulus.

Question 37

How can Young modulus be found from a graph?

Answer 37

The gradient of a stress-strain graph.

Question 38

How are stiffness and strength different?

Answer 38

A stiff material is one that is difficult to stretch/compress

A Strong material is one that has a high breaking stress

A material doesn’t have to be strong if its stiff, these features aren’t linked to each other

Question 39

What would a stress-strain graph with a high gradient with a high breaking stress show?

Answer 39

The material is stiff and strong.

Question 40

What would a stress-strain graph with a high gradient with a low breaking stress show?

Answer 40

The material is stiff and weak.

Question 41

What would a stress-strain graph with a low gradient line with a high breaking stress show?

Answer 41

The material is less stiff and strong.

Question 42

What would a stress-strain graph with a low gradient line with a low breaking stress show?

Answer 42

The material is less stiff and weak.

Question 43

Describe the “CPAC 4: Use a falling ball method to determine the viscosity of a liquid.” experiment.

Answer 43

1) Fill a wide, clear tube with the liquid you want to investigate (you need to know its density).

2) Put one rubber band about halfway down the tube at a position such that the ball bearings will have achieved terminal velocity when they reach it.

3) Place two more plastic bands below the first so that the distance between each band is equal and the lowest band is near the bottom of the tube. Measure the distance between bands using a ruler.

4) Measure the diameter of each ball bearing using a micrometre and halve it to get the radius. Also calculate their densities using mass / (4/3)πr^3.

5) Drop the ball bearing into the tube. Start two stopwatches when the ball reaches the first band, and record the time at which it reaches the first band with the first stopwatch and then the time at which it reaches the second band with the second stopwatch.

6) If the ball falls close to the wall, redo the reading because the flow will no longer be laminar and therefore Stoke’s law will no longer apply.

7) Repeat this at least three times for each ball bearing to reduce the effect of random errors in your results. You can use a strong magnet to retrieve the ball bearings from the tube. Then repeat this for the different sizes of ball bearings.

8) Calculate the average time taken for each size of ball bearing to fall between the first and second band (t1) and then the second and third band (t2 - t1). Use the average time and the distance between bands to calculate the average (terminal) velocity of the ball bearing between the elastic bands.

9) You can then calculate the viscosity, “η”, of the liquid. The ball bearings are falling at terminal velocity, so there the resultant force acting on the ball is zero: W - D - U = 0

Substituting the equation for each force and rearranging gives the formula for viscosity:

η = (2(r^2)(P(solid) - P(liquid))g) / 9v

10) Use the different values for viscosity from each of the different sizes of ball bearings to calculate an average viscosity.

Question 44

Describe the “CPAC 5: Determine the Young modulus of a material.” experiment.

Answer 44

1) The test wire should be thin and as long as possible because the longer and thinner the wire, the more it extends for the same force which reduces the percentage uncertainty in the measurements.

2) Find the diameter of the wire three times at three different points using a micrometre and take an average of the measurements. Assuming the cross-section is circular, use the formula π(d/2)^2 to determine its cross-sectional area.

3) Clamp a pulley attached to the wire to the bench so you can hang weights off one end of it. Put the smallest weight necessary to straighten the wire at the end of it.

4) Measure the distance between the fixed end of the wire and the marker, this is your unstretched length.

5) As you increase the weight, the wire stretches therefore the wire moves.

6) Increase the weight in equal steps (e.g. 100g intervals), recording the marker reading each time by finding the difference between the newly extended length and the original length.

7) Use your results to calculate the stress of the wire (σ = F / A) and strain (ε = extension, “δ” / L). The young’s modulus is the gradient of the line.

Question 45

What is another way of finding the young’s modulus of a material.

Answer 45

Using Searle’s apparatus.